The group behind what’s expected to be the most expensive and sustained assault ― a super PAC dedicated to Rubio called Conservative Solutions PAC ― has raised about $20 million in the past week alone, sources tell POLITICO. They say the cash will power a full-frontal assault on Trump in the delegate-rich states that vote in March, starting with Tuesday’s 14 Super Tuesday contests.

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Taken together, the wave of big-money attack ads, which could total in the eight-figures, suggests that deep-pocketed conservative groups and their donors see the March contests as their last chance to stop the billionaire real-estate showman from winning the GOP nomination and taking over the party.

It’s a campaign that operatives and Republican Party leaders had been encouraging for weeks, and they’ve expressed frustration that none of the deep-pocketed groups on the right had been stepping up to wage it.

The hesitance stemmed at least partly from the super PACs devoted to Trump’s rivals. The operatives running those PACs seemed unwilling to spend serious money lashing Trump even when their preferred candidates went after the Manhattan billionaire. That was the case with the super PAC backing Jeb Bush, which spent a tiny fraction of its $119-million haul criticizing Trump, despite repeated, if ineffective, swipes at Trump by the former Florida governor, who has since dropped out of the race.

It’s unclear whether the wave of ads launched Friday ― combined with Rubio’s sudden aggressiveness towards Trump ― will be too little, too late. Trump maintains commanding leads in many of the states that vote on Super Tuesday and beyond, even boasting a wide lead in Rubio’s home state of Florida headed into the state’s March 15 primary, where a loss would be devastating for Rubio.

But Rubio’s Thursday night performance was welcomed by establishment donors and operatives alike as a sign that he was their best hope to stop Trump’s momentum, and also as revealing the blueprint for doing so. And several sources involved in the super PAC attacks launched Friday suggested their decisions to act now likely derived from similar assessments of the calendar and delegate math as the one that prompted Rubio and his rival Ted Cruz to tear into Trump on Thursday night. The attacks left the front-runner looking flustered and defensive for perhaps the first time in a campaign that he had mostly controlled.

Trump “hasn’t faced a sustained assault across the full spectrum of his vulnerabilities: his left-wing views on policy, his unsavory business practices, and his scandalous personal life,” said an operative involved in the big-money attacks on Trump. “Maybe none of that will move the needle, but we will get a chance to find out before the Democrats do.”

The pro-Rubio super PAC Conservative Solutions unveiled two Trump attack ads early Friday morning. One hit the billionaire for his foreign policy inexperience and unwillingness to more stridently side with Israel over the Palestinians. The other cast Trump as a greedy businessman who “uses sleazy bankruptcy laws to avoid paying workers” and who is playing Republican voters for “fools.”

Those lines of attack parallel taunting riffs that Rubio delivered against Trump to great effect in Thursday’s debate on CNN. The Florida Senator picked right back up on Friday morning, seemingly taking great pleasure in Trump’s angry pushback after the debate, which included misspelled tweets (since-deleted) that blasted Rubio as a “chocker,” and “leightweight."

Rubio repeatedly called the billionaire businessman a “con artist” on Friday, first on the morning shows and then at a Dallas rally, where he also suggested Trump may have wet his pants during the debate. Conservative Solutions picked up the con-artist line, and sources who work with major donors told POLITICO that the donors can’t get enough of the new offensive.

The super PAC has already purchased or reserved $6.4 million worth of ads in the week leading up to Super Tuesday and the week following it, according to an analysis of advertising buys for POLITICO by The Tracking Firm. By comparison, Trump’s own campaign has only committed less than $900,000 to ad spending in the Super Tuesday states.

Yet, puzzlingly, a handful of super PACs devoted to boosting Cruz’s campaign still appeared to be shying away from aggressively taking on Trump, despite the Texas senator’s own aggressive efforts to take on Trump.

A network of pro-Cruz super PACs called Keep the Promise, on Friday afternoon debuted a $2.4 million Super Tuesday campaign comprising a series of hagiographicpositiveads that did not once mention Trump, Rubio or even Democratic presidential candidates Hillary Clinton and Bernie Sanders.

That prompted puzzlement among some Cruz supporters, who had hoped the super PACs would follow Cruz’s lead. Some suggested that the disconnect between the campaign and the super PACs reveals the difficulty posed by federal coordination restrictions, which bar campaigns and big-money groups from working together on advertising strategy. Though Cruz has been ramping up his attacks on Trump for weeks, so it shouldn’t have been hard for the super PAC operatives to follow suit.

Another Cruz super PAC called Stand for Truth took a slightly sharper approach, featuring clips of Trump, Clinton and Sanders, while warning that, "with so much at stake, we can't afford to take a chance on this election." But the ad's most direct swipe comes at Rubio, over his immigration policy.

But the ad, backed by a $1-million buy in four Super Tuesday states, seems like patty cake next to the testy back-and-forth on the debate stage Thursday.